1. The Field of the Invention
The present invention relates generally to methods and devices for coupling and mounting optical components. More particularly, embodiments of the present invention relate to an improved device, as well as to its manufacture and use, for coupling fiber optic components in such a way as to ensure adjacent contact between the coupled components and thus facilitate high levels of thermal stability in the completed optical assembly.
2. The Prior State of the Art
Traditionally, data and signal transmission has been accomplished by sending a series of electronic pulses along one or more metallic wires or cables to a receiver, which then converts the pulses into a usable form. Within a limited range, metallic wires and cables are generally effective as a signal and data-carrying medium. However, metallic wires suffer from a variety of shortcomings which serve to limit their effectiveness as a transmission medium.
First, all metallic wires and cables are characterized by imperfections such as chemical impurities. These imperfections cause the electronic pulses to lose energy, or attenuate, as the pulse travels down the wire. As a result, signal regenerators are required at various points along the wire to electrically boost the electronic pulses and thus effect transmission of a signal and/or data.
Second, the amount of information that a metallic wire or cable is capable of carrying, i.e., the bandwidth of the wire, is constrained by limitations inherent in the wire. In particular, bandwidth is sharply limited by factors including, but not limited to, the size and composition of the transmission wire.
The bandwidth limitations inherent in metallic wire has become increasingly problematic as the existing wire networks of telephone companies are called upon to transport increasing amounts of voice and data signalsxe2x80x94especially with the proliferation of the Internet. Moreover, wires and cables take up a great deal of space in already crowded wireways and trunks, and are difficult to work with and install. They are also susceptible to electromagnetic interference, voltage surges, and similar types of electronic noise. Thus, there has been an increasing need for alternative forms of technology to meet data and signal transmission needs.
One increasingly popular approach is to utilize fiber optic technology. Essentially, fiber optic technology employs optical waveguides made of glass, plastic or the like, as a means for transmission of data and/or signals in the form of light pulses. Typically, optical waveguides take the form of optical fibers. There are a number of advantages in using fiber optics as a data and signal transmission medium: fiber optics were relatively lighter in size and weight as compared to metallic transmission mediaxe2x80x94some experts have estimated that it would take 33 tons of copper to transmit the same amount of information handled by 4 ounces of optical fiber; optical fibers were not susceptible to electromagnetic interference and voltage surges; optical fibers were less prone to signal attenuation; and, perhaps most importantly, optical fibers possessed a tremendous bandwidth.
As a result of the numerous advantages associated with the use of fiber optic technology as a means for data and signal transmission, fiber optics are being used in an ever-increasing number of applications, including local area networks (LANs) and a variety of communications systems. Also, the ascendancy of the Internet has served to emphasize the need for a transmission medium capable of transmitting large amounts of data over great distances both rapidly and reliably. Fiber optic technology is well suited to serve this end because of its large bandwidth capabilities, high data transmission speedsxe2x80x94up to 1600 times faster than conventional copper wires, and relatively low signal attenuation characteristics.
Many examples of the implementation of fiber optic technology can be found in the telecommunications industry. Some examples of widely used fiber optic arrangements include wavelength division multiplexing (WDM) devices, and gain-flattening devices.
Clearly, fiber optic technology represents a significant advance in the field of data and signal transmission. However, despite the numerous advantages of fiber optic technology and the continuous advances being made, there are still a variety of problems in the field that are as yet unresolved. As indicated in the following discussion, one of the major unresolved problems in the fiber optics field is the thermal instability of many fiber optic assemblies.
In general, thermal instability refers to the inability of an optical assembly to consistently perform in accordance with a desired set of specifications when exposed to a particular range of temperatures. For example, an optical assembly that performs in accordance with the desired specifications at room temperature will often be xe2x80x98out of specxe2x80x99 at elevated operating temperatures. Specifically, relatively high operating temperatures cause the various components in typical optical assemblies to change their orientation and/or position with respect to each other. Because the precise positioning and alignment of the optical components is critical to the effective performance of an optical assembly, any movement or shifting of the individual optical components compromises the performance of the optical assembly as a whole. The problem of thermal instability is particularly acute in the area of micro-optics where interference filters, for example, are generally as small as 4 mm2, or less, in cross-sectional area.
Though thermal instability can be thought of in terms of relative spatial movements or shifts of optical components in an optical assembly, practitioners in the art have found it convenient to express thermal instability in a more precise fashion. Specifically, it is generally acknowledged that thermal stability may be described in terms of the shift of the center wavelength (CWL) of the passband as a function of temperature; wherein the CWI shift is expressed in units of picometer/xc2x0C., or pm/xc2x0C., and xe2x80x98passbandxe2x80x99 refers to a range of wavelengths desired to be transmitted, or passed, through a given interference filter. As an example, the CWL of a known 50 GHz filter is about 1.1 pm/xc2x0C. over an operating range of 0-80xc2x0 C. 
Thermal instability in optical assemblies very often stems from the manner in which the individual optical components are assembled and/or held in place. A typical optical assembly employs one or more gradient index (GRIN) lenses, one or more optical fibers, and at least one interference filter. Often, these components are attached to each other in a face-to-face configuration and retained in place by means of epoxy or other adhesives applied to the respective faces. The adhesive typically has an index of refraction substantially the same as the attached optical components. In other cases, the optical components are retained in a spaced-apart configuration by means of adhesive in combination with some type of structural mount.
In operation, transmitted light travels down the optical fiber to the GRIN lens, which then collimates the transmitted light. The collimated light is thus refocused by the GRIN lens for transmission through another optical fiber. Typically, an interference filter is attached to, or near, the GRIN lens so as to transmit and/or reflect only selected wavelengths of the collimated light leaving the GRIN lens.
Although widely used, the known assembly methods and mounting structures for optical components contribute significantly to the thermal instability of optical assemblies. One feature of known mounting methods that is particularly problematic is the application of adhesives or the like to the faces of optical components so as to fasten optical components together in a face-to-face configuration. The problems arising from such methods are cause for concern in any optical assembly, but are of particular concern where the method is used to attach a thin film interference filter to a GRIN lensxe2x80x94a common arrangement in both transmissive and reflective fiber optic devices.
One objective in optical design is to minimize the number of non-essential components in the optical path because non-essential components can distort, impede, and disrupt light transmission. Obviously, introduction of adhesive between optical components is at cross purposes with this objective. Adhesives are not essential to performance in the sense that they have some desired effect on the transmitted light, rather, they simply serve to hold optical assemblies together. Use of adhesives with indices of refraction matched to those of the joined optical components is simply an attempt to minimize the negative effects of the adhesive.
Another major problem with the use of adhesives and the like in the optical path concerns the rate of thermal expansion of the adhesives. In particular, the adhesives commonly used to join optical components in face-to-face configurations generally have thermal expansion rates many times greater than the thermal expansion rates of the joined optical components. The practical effect of this differential in thermal expansion rates is that, when exposed to elevated operating temperatures, the adhesive expands further and more quickly than the optical components on either side of it. Thus, the optical components are moved and shifted by the rapidly expanding adhesive. As previously noted, movement and shifting of optical components, relative to each other, degrades the performance of the optical assembly. Furthermore, if the adhesive is not applied uniformly to the joined components, as is often the case in the typical manually assembled optical device, the expansion of the adhesive will be non-uniform as well; that is, some portions of the adhesive may expand a greater distance than others, thereby causing the joined optical components to tilt and move out of alignment with each other. Obviously, non-uniform and uncontrolled expansion of the adhesive is an undesirable result given the critical importance of positioning and alignment of optical components in optical assemblies. Note that the problems caused by introduction of adhesive in the optical path become particularly acute where the joined optical surfaces are not flat, e.g., where one surface is convex and one is concave.
Other known optical assemblies do not use adhesives on the faces of joined optical components, rather, adhesive is applied to the edges of the optical components so as to facilitate attachment of the components to a mount. However, practical considerations make this approach problematic as well.
In particular, the edges of the optical components form an interface, or contact area, between the optical component and the mount. Further, the optical components also form a contact area with each other where they touch; it is inherent in the geometry of such an arrangement that this face-to-face contact area is coterminous with the contact areas formed between the respective optical components and the mount. Accordingly, when adhesive is applied to the edges of optical components, so as to secure those components to the mount, the adhesive tends to wick by capillary action into the face-to-face contact area. In other arrangements, the two optical components are brought into face-to-face contact, and adhesive is applied around the periphery of the contact area. However, wicking of the adhesive commonly occurs when this assembly method is used. Specifically, the adhesive moves by capillary action from the periphery of the contact area into the contact area itself, and thus into the optical path. For at least the reasons previously discussed, it is highly undesirable to have adhesives or other non-essential components in the optical path.
Use of adhesives on or near the faces of joined optical components is problematic for other reasons as well. For example, in the event of an amplifier overload, such as might be experienced in the context of communications system operations, the adhesive in the optical path could fail and render the optical system inoperative.
Finally, any adhesive used to join the faces of optical components must be able to transmit light in the wavelength range of interest. Accordingly, there is a limited universe of adhesives from which to selectxe2x80x94such specialized adhesives necessarily add to the cost of optical assemblies.
Some attempts have been made to resolve the problems realized when adhesive is introduced into the optical path. In one approach, the optical components are made relatively long, so that adhesive applied to the edges of the components that are in contact with the mount is less likely to wick into the contact area between the joined optical components. While somewhat effective in reducing the wicking problem, this approach has the drawback that relatively large areas of the components are unconstrained by adhesive and are thus free to move in response to changes in operating temperature. As noted earlier, movement of the optical components has a substantial and undesirable effect on the performance of the optical assembly.
At least one other attempt has been made to foreclose the problem of adhesive wicking into the optical path. The typical solution has been to interpose a gap or gaps between the optical components in an optical assembly. The optical components are then joined to a mount by means of adhesive applied to the edges of the components. Since the optical components do not contact each other in these arrangements, there is little danger of adhesive wicking into the optical path. However, these types of arrangements create another set of problems.
A major shortcoming of spaced-apart arrangements wherein gaps are introduced between the optical components is that the gaps have a detrimental effect on optical performance. As previously noted, it is generally acknowledged that optimum optical performance is achieved when the optical components are in direct contact with each other. When the components are spaced apart, transmitted light can reflect off the surfaces of the optical components; this is an undesirable result because the reflected light tends to disrupt light transmission and thus degrade the performance of the optical assembly. Further, reflection off the optical surfaces also causes interference fringes which act to disrupt and degrade light transmission.
Finally, one other significant problem associated with spaced apart arrangements of fiber optic components in an optical assembly is that because the optical components are not in direct contact, it is difficult to ensure that, in the final assembly, the optical components are properly aligned with respect to each other, i.e., not tilted. The already difficult task of aligning components in these types of assemblies, particularly where micro-optics are concerned, is further exacerbated by the fact that these optical assemblies are typically assembled by hand. Manual assembly techniques introduce an element of inconsistency where the arrangement and orientation of the optical components are concerned.
In view of the foregoing problems with the known methods and devices used to couple fiber optic components, what is needed is an improved bonding device and a mounting method for mounting optical components in the bonding device that will ensure stable performance of fiber optic assemblies over a wide range of operating temperatures. Specifically, the improved bonding device should be configured to facilitate a variety of physical arrangements between optical components, and should be able to accommodate optical components having a variety of sizes and cross-sectional shapes. Additionally, the improved bonding device should ensure alignment between optical components when the components are fully seated in the coupling. Further, the improved bonding device should be configured to prevent wicking of bonding agents into the face-to-face contact area between the coupled optical components. Also, the bonding device should be composed of a material having thermal properties similar to those of the coupled optical components so as to minimize the adverse effects of non-uniform expansion of the optical assembly.
With regard to mounting of the optical components in the bonding device, the improved method should be useful with a variety of bonding media so as to permit optimization of the thermal and mechanical characteristics of the optical assembly. Further, the improved mounting method should ensure that the bonding device exerts a force on the mounted optical components which will tend to maintain contact, or other desired orientations, between the components when the assembly is heated to operating temperatures. Also, the improved mounting method should overcome any tendency of the adhesive to separate the optical components during the mounting process. Finally, the improved mounting method should be readily automated.
The present invention has been developed in response to the current state of the art, and in particular, in response to these and other problems and needs that have not been fully or completely solved by currently available fiber optic component mounting methods and couplings. Thus, it is an overall object of the present invention to provide an improved bonding collar and mounting method, utilizing the bonding collar, that are particularly useful for joining fiber optic components. It is another object of the invention to provide a bonding collar that introduces a space between the areas where the respective optical components are bonded to the bonding collar, so as to prevent adhesive from wicking into the face-to-face contact area between the optical components. It is yet another object of the invention to provide a bonding collar that is capable of coupling substantially round GRIN lenses with substantially square thin film interference filters, as well as being adapted to receive optical components of disparate sizes. A further object of the invention is to provide a bonding collar having a coefficient of thermal expansion that falls within a predetermined range.
It is another object of the present invention to provide an improved optical component mounting method that is effective with a variety of bonding agents, including solder, adhesives and the like. It is a further object of the present invention to provide a mounting method that helps ensure that the desired positions of the optical components are properly maintained during the curing of the bonding agents.
Finally, it is an object of the present invention to provide a mounting method that helps ensure that the optical components in the optical assembly produced by the inventive mounting method will maintain their positions when the optical assembly is exposed to thermal variations during operation.
In summary, embodiments of the present invention are directed to an improved bonding collar and mounting method, utilizing the bonding collar, for use in coupling fiber optic components. Presently preferred embodiments are particularly suitable for use in micro-optic systems where it is desired to join thin film interference filters to GRIN lenses. Typically, the optical assemblies thus formed find particular application in a variety of communications systems, including telephone systems, and computer network applications.
In a preferred embodiment, the bonding collar is composed of a material or combination of materials with thermal properties similar to those of the GRIN lens and filter. Preferably, the bonding collar material is 416 SS, 303 SS, or the like. In a preferred embodiment, the bonding collar is formed by drilling a concentric first socket partway into one end of a cylindrical blank and then cutting four slots in the other end of the cylindrical blank, wherein the slots are perpendicular to the end of the cylindrical blank and each intersects the first socket, so that upon cutting the fourth slot, a waste portion of material drops out, leaving a square second socket in communication with the concentric first socket.
In presently preferred embodiments, the bonding collar is preferably used to couple a thin film interference filter having an approximately 4 mm2 face with a GRIN lens approximately 1.8 mm in diameter. In one embodiment, the GRIN lens is received in the concentric first socket so as to form a first contact area between the GRIN lens and the bonding collar, and the filter is received in the square second socket so as to form a second contact area between the filter and the bonding collar. Also, the GRIN lens and filter are preferably in a face-to-face contact orientation, so as to form a third contact area, when each is fully received in the bonding collar. In a preferred embodiment, the bonding collar is configured so as to ensure that the third contact area is noncoterminous with respect to the first and second contact areas. By thus separating the first and second contact areas from the third contact area, the space serves to prevent transportation of adhesive, via capillary action, from the first contact area between the GRIN lens and the bonding collar to the third contact area of the GRIN lens and filter. The space also serves the same function with respect to the second contact area between the filter and the bonding collar.
Preferably, the GRIN lens is bonded to the bonding collar with a bonding agent, adhesive, or the like. For example, the adhesive is applied so as to be equally spaced around the visible interface between the GRIN lens and the bonding collar after the GRIN lens has been cleaned and inserted into the bonding collar. In a preferred embodiment, a plurality of bonding collars are arranged in a fixture, which comprises a plurality of pins or the like adapted to fit into the respective square sockets of the bonding collars. The plurality of bonding collars are placed onto the pins and oriented with their respective concentric holes facing upwards so as to receive the respective plurality of GRIN lenses. Preferably, the fixturing of the collars, insertion of the GRIN lenses, and subsequent application of the adhesive are automated. The GRIN lensxe2x80x94bonding collar subassemblies thus formed are placed in an oven or the like for curing of the adhesive. In one illustrative embodiment, the adhesive is cured for about 10 minutes at a temperature of about 110xc2x0 C., then the GRIN lensxe2x80x94bonding collar subassemblies are removed from the oven and allowed to cool.
In a preferred embodiment, the plurality of completed GRIN lensxe2x80x94bonding collar subassemblies are then fixtured in a filter bonding jig comprising a plurality of holes, the holes being slightly larger in diameter than the GRIN lenses so as to securely receive the plurality of the GRIN lenses, the respective square sockets of the plurality of bonding collars facing upwards. A thin film interference filter is then placed in each of the bonding collars so that the film side of the filter completely contacts the upward facing surface of the GRIN lens. In a preferred embodiment, the respective contacting surfaces of the GRIN lens and the filter are complementary planar surfaces. Adhesive can be applied to the visible interfaces between the top of the bonding collar and the sides of the filter. In a preferred embodiment, a weight centering sleeve is then lowered over each of the filterxe2x80x94GRIN lens assemblies. For example, a shaft with a weight at one end and a plastic pad at the other is inserted into the weight centering sleeve until the plastic pad contacts the filter, the plastic pad serving to hold the filter securely against the face of the GRIN lens.
Preferably, the assemblies thus formed are then cured in an oven or the like. In one presently preferred embodiment, the curing process comprises the steps of: i) curing the assemblies for about 10 minutes at about 110xc2x0 C., then removing the assemblies and allowing them to cool; ii) removing the weights, sleeves, and bonded assemblies from the filter bonding jig and placing the bonded assemblies in a curing fixture, the curing fixture then being placed in an oven so as to further cure the bonded assembliesxe2x80x94preferably, this curing step comprises curing at about 75xc2x0 C. to about 85xc2x0 C. for about 1 hour; and, iii) removing the curing fixture from the oven and placing the curing fixture in another oven to cure at about 105xc2x0 C. to about 115xc2x0 C. for about 1 hour, and then removing the bonded assemblies and allowing them to cool. In a preferred embodiment, the mounting method described herein results in a CWL shift of the optical assembly, during operation, of about 0.1 pm/xc2x0C. to about 0.25 pm/xc2x0C.
These and other objects, features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.